Many communication protocols exist and are known for conveying data between communication devices. Many of these protocols address the problem of data corruption which can occur during any data communication. In wireless communication systems, for example, the problems of data corruption are particularly acute. In such systems, automatic repeat request (ARQ) protocols are often used in order to achieve some level of reliable data transfer. In ARQ protocols, when corrupted data is received, the receiving device requests the sending device to resend the corrupted data. For instance, a sending device sends a stream of data divided into data blocks to a receiving device. Through error detection coding, the receiving device may detect that the data in some of the data blocks has been corrupted. When such corruption is detected, the receiving device then responds with an acknowledgment (ACK) that indicates which of the data blocks were corrupted. The sending device may then resend the corrupted data blocks in an attempt to successfully convey the data.
Merely resending the data, however, may not be enough. In the article "Data Capacity of TDMA/FDMA Systems with Adaptive Modulation", the technique of resending corrupted data at a reduced data rate is discussed. Reducing the data rate may enable the receiving device to correctly distinguish the data from background noise or interference, thereby increasing the likelihood of a successful data transfer. In such a protocol, the receiving device indicates in its ACK which data blocks contain corrupted data. The corrupted data blocks are then resent by the sending device at a lower data rate. If some of the resent data blocks are still corrupted, such data blocks are resent at yet a lower data rate. This technique of reducing the data rate continues until all the data is successfully transferred or a minimum data rate is reached. By reducing the data rate only when data corruption occurs, these rate reduction protocols attempt to use as little communication bandwidth as is necessary to successfully and expediently transfer data.
One problem with a rate reduction data protocol, is the use of too much communication bandwidth in certain situations. In a wireless communication system, for instance, a number of subscribers approaching the edge of a coverage area may be communicating on very poor quality channels. Use of the above described rate reduction protocol between the subscribers and the communication system infrastructure may quickly result in many transmissions at the lowest data rate. Thus, to transmit the same amount of data as subscribers communicating on high quality channels, the subscribers communicating on low quality channels consume much more transmission time, due to transmitting at a lower data rate, and thereby utilize a greater portion of the available bandwidth. With a number of such subscribers utilizing a greater and greater portion of the bandwidth, less and less bandwidth is available for the rest of the subscribers. The overall performance of the communication system suffers since a greater portion of the available bandwidth is being utilized for lower data rate transmissions.
Therefore, a need exists for a method and apparatus of conveying data reliably, while utilizing bandwidth more efficiently.